Containers of plastic material have steadily gained ground in employment for the storage of different products and, in particular, the storage of foods. Such containers must meet a number of requirements, including that they be easy to open and generally provide reliable and tight reclosure, that they guarantee reliable sealing as in factory-sealed containers, and also provide tightness against entry of micro-organisms, and that the containers possess the barrier properties requisite for their contemplated application. The requirement of tightness against micro-organisms such as, for example, bacteria or other pathogenic organisms must be met even if the requirement entails problems in design, filling and original--or factory--sealing of the containers. In certain fields of application, these difficulties are further aggravated by the fact that the containers must be acceptable for hot filling and/or must be able to withstand heat-treatment after filling, for example for pasteurization or autoclaving. In particular in heat-treatment, the connection between the container body proper and its seal is, in a factory-sealed container, exposed to great stresses which may lead to leakage. During cooling of the heat-treated container with its contents, pressure differences arise, moreover, between the interior of the container and its ambient surroundings, which, at least under certain phases of the cooling process, entails that a partial vacuum prevails in the container, with a subsequent risk that micro-organisms can be drawn into the container even for the very smallest leaks in the closure.
In order for a new container to be approved as suitable for use for long storage life of foods, for example, conserved and preserved foods, it is required that a large number of container samples of the type under consideration be manufactured and scrutinized. Such scrutiny relates also to the long-term tightness of the containers and it is, therefore, a costly and substantial procedure before a new type of container can be put out on the market. It is necessary that such a scrutiny for a new type of packaging comprise at least 100,000 containers in order to provide a reliable basis for an assessment of the quality and tightness of the new type of packaging. It is also obvious that such careful scrutiny is particularly necessary for packagings intended for the long-term storage of foods, since any degree of untightness against bacteria and micro-organisms would result in deadly consequences for the consumer of foods attacked by pathogenic or toxicogenic micro-organisms, particularly under such unfortunate circumstances that the taste of the food involved is not influenced to such an extent as to forewarn against consumption.